The present invention relates to acoustic projectors, especially projectors for use in low frequency military and civilian sonar systems, and in particular to underwater acoustic projectors having highly stable performance with depth and reduced manufacturing costs due to lower mechanical tolerances being required than in existing acoustic projectors.
Low frequency military and civilian sonar systems require compact, light weight, high power, efficient, wide bandwidth acoustic projectors whose performance is stable with depth and linear with drive voltage levels and which have a low manufacturing and maintenance cost.
Canadian Patent 1,319,414 by Bryce Fanning et al that issued on Jun. 22, 1993 describes one type of a free-flooding piezoelectrically driven resonate-pipe projector (RPP) with vent holes in the pipe walls to broaden the response of certain cavity resonances and to increase the response between those resonances. The drive unit is a radially-poled lead zirconate-titanate cylinder with aluminum pipes extending into the center of the piezoelectric drive unit, the pipes being mechanically coupled to the drive unit. To accomplish the necessary acoustic coupling between the drive unit and pipes requires a close mechanically fit to couple the drive unit to the pipes. These resonant pipe projectors are partially free-flooding and can be operated at extreme depths because the drive unit is highly resistant to hydrostatic loading. However, the bandwidth is small and they are expensive to manufacture due to the high degree of tolerances required.
Flextensional projectors are amongst the best ones presently available to meet the military and civilian sonar systems requirements, one of the most promising flextensional projectors being the barrel stave type. The barrel stave projector (BSP) is a compact, low frequency underwater sound source which has applications in low frequency active (LFA) sonar and in underwater communications. In one known BSP design, such as described in U.S. Pat. No. 4,922,470 by G. McMahon et al, a set of curved bars (staves) surround and enclose a stack of axially poled piezo-electric rings. The staves act like a mechanical transformer and help match the impedance of the transducer to the radiation impedance of the water. Axial motion of the stave ends is transformed to a larger radial motion of the stave midpoints. This increases the net volume velocity of the water, at the expense of the applied force, and is essential for radiating effectively at low frequencies.
This known BSP projector has slots between the staves which are required to reduce the hoop stiffness and achieve a useful transformer ratio. However, these slots must be waterproofed by a rubber membrane (boot) stretched tightly and glued with epoxy around the projector. This boot also provides effective corrosion protection for the A1 staves. However, the variation in performance with depth of the BSP is suspected to depend in part on the boot. At increasing depths, hydrostatic pressure pushes the boot into the slots causing the shell to stiffen tangentially, increasing the resonance frequency, and causing an increasing loss of performance. This depth sensitivity of a barrel stave projector can be reduced somewhat by reinforcing the boot over the slots. It is also possible to pressure compensate the BSP with compressed air or other gas resulting in good acoustic performance at greater depths.
The slots in the BSP, as a secondary effect, provide a nonlinearity in the response of the projector to hydrostatic loading. The staves will deflect inwards together under increasing hydrostatic loading (assuming no pressure compensation) since the projector is air filled. Depending on the thickness and stiffness of the rubber, it is reasonable to expect that as the slots close at great enough depths, that closure of the slots due to increasing depth will force the boot back out of the slots. The projector will now be very stiff and resistant to further effects of depth until the crush depth of the now, effectively, solid shell is reached. This provides a safety mechanism which may save the projector in case an uncompensated BSP is accidentally submerged very deep or a pressure compensation system runs out of air.
Variants of this known BSP have been built to optimise light weight, wide bandwidth, low frequency, high power, and improved electroacoustic efficiency. Efficiency is an especially critical parameter for the high power versions of the BSP because the driver is well insulated from the water thermally. The boot""s relatively poor thermal conductivity contributes to the difficulty in cooling the BSP.
There is evidence that the interelement variability in performance amongst a set of 20 of these projectors used in a horizontal line array was due largely to variability in the boot""s material properties Most of these projectors subsequently failed due to chemical incompatibility of the boots with the hydrocarbon-based towed-array fill fluid, underscoring the need for consideration of chemical compatibility whenever elastomer clad projectors are exposed to fluids other than seawater. The neoprene boot is a potential weak point for the BSP in terms of damage due to rough handling. Even a pinhole in the boot can lead to projector failure by flooding. Overhaul of a barrel stave projector usually involves boot replacement. The cost of a custom molded neoprene boot is approximately $20.00 but the labour cost of installing the boot is typically several person hours spread over 2 days (of glue curing time) contributing to the relatively high maintenance cost for these BSPs.
The inside surfaces of the (eight) staves of these BSPs are machined individually from bar stock on a numerically controlled (NC) milling machine. The staves are then mounted together on a fixture and the outside surfaces are turned on a tracer lathe. The machining and handling costs are such that the staves are the most expensive parts of the BSP. These BSPs are, as a result, both relatively costly to manufacture and maintain.
Since the radiating surface of this BSP is waterproofed with a rubber membrane, it is susceptible to chemical attack and degradation and damage due to flooding through pinholes. The BSP suffers from variation of performance with depth caused by water pressure forcing the rubber membrane into the slots between the vibrating staves of the projector unless a pressure compensation system is fitted. The BSP shows nonlinearity of performance versus drive voltage due to effects of the rubber membrane. Thus there could be substantial advantages to accrue if it were possible to develop a one-piece flextensional shell for the BSP that does not require a boot.
A one-piece flextensional shell projector is described by Christopher Purcell in U.S. Pat. No. 5,805,529. The surface of this projector is formed of a thin-walled one-piece inwardly concavely shaped shell containing corrugations running in the axial direction. This one-piece shell is slotless which eliminates the requirement for a boot.
It is an object of the invention to provide an acoustic projector with reduced depth sensitivity when submerged in water, improved efficiency and reduced manufacturing costs.
An acoustic projector, according to one embodiment of the present invention, comprises a pair of spaced apart end walls with an acoustic driver positioned between and connected to the end walls, the driver having smaller cross-sectional dimensions than the end walls which have tubular pipe waveguides extending outward from the driver, outer ends of the waveguides being open.